1. Field of the Invention
The present invention relates to user-programmable antifuse technology. More particularly, the present invention relates to metal-to-metal antifuse elements and to devices and circuits for protecting metal-to-metal antifuses from the effects of electrostatic discharge during the fabrication process which produces them.
2. The Prior Art
During the fabrication process used for manufacturing circuits including antifuse elements, the integrated circuit wafer is exposed to environments which are potentially harmful to the antifuse material positioned between the lower and upper antifuse electrodes. For example, reactive ion plasma etching techniques employed to anisotropically define small geometry features can result in the accumulation of large static charges giving rise to high voltages across sensitive insulating areas in the integrated circuit being fabricated.
Antifuse materials, including dielectric materials, which are engineered to rupture at predictable voltages, are particularly sensitive to this phenomena. The voltages generated by static charge buildup can easily exceed the programming voltages of the antifuse devices and can thus inadvertently disrupt the antifuse material during processing and destroy the device by programming it. Where the antifuse material utilized is amorphous silicon, silicon dioxide or silicon nitride it is of vital importance to protect the antifuse material layer from damage during the plasma etch step defining this layer. If the antifuse structure is not protected from static charge buildup during plasma etch, a potentially high number of defects will appear in the antifuse material in the form of short-circuited antifuse circuits, since charge buildup can only be released through the antifuse material layer.
One structure for protection of antifuses during processing is disclosed in U.S. Pat. No. 4,941,028 to Chen et al., which teaches the use of both active MOS transistor structures and deliberately-thinned dielectrics formed over a lightly-doped region in a substrate to protect antifuse material comprising thin dielectric structures during semiconductor processing steps which form the antifuses.
It is not always possible, however, to provide static-charge protection schemes which rely on active devices or doped regions in a semiconductor substrate as do the solutions presented by Chen et al. For example, when the insulating layer to be protected is an antifuse material layer in an antifuse device disposed between two metal-interconnect layers in a microcircuit structure such as an integrated circuit, it may not always be practicable to provide a static-charge protection device which relies on active transistor regions occupying space in a semiconductor substrate. Accordingly, it would be advantageous to provide a static-charge protection circuit to protect antifuses disposed between metal interconnect layers.